This invention relates generally to photocatalytic reactors and more particularly to a photocatalytic reactor having at least one permeable photocatalytic membrane. The United States Government has rights to this invention pursuant to Contract No. DE-AC05-840R21400 with Lockheed Martin Energy Systems, Inc. awarded by the U.S. Department of Energy.
The use of photocatalysis for in situ environmental remediation and other applications is a growing area of research. In the process of photocatalysis generally, the illumination of the surface of a photocatalyst, such as an oxide semiconductor, produces chemically active sites associated with excited electron-hole pairs. The positive holes migrate to the semiconductor surface and participate in oxidation reactions, while the electrons are scavenged by an oxidizer such as oxygen. The complete mineralization (reduction to CO.sub.2, H.sub.2 O, and halogen ions) of many halogenated organics such as trichloroethylene has been shown to proceed by these oxidation-reduction reactions at such active sites on the photocatalyst surface.
In photocatalysis, the molecule to be changed must contact or closely approach the photocatalyst surface. Most photocatalysis research to date has used slurries of small titanium dioxide particles, mixed with the dissolved substances of interest, in order to keep the titanium dioxide photocatalytic surfaces as close as possible to the molecules dissolved in the slurry. However, because subsequent recovery of particles of photocatalyst from a treated liquid can be difficult and creates an additional step, various methods for immobilization of the photocatalyst have been attempted. Such methods include coating the photocatalyst on glass beads, inside tubes of glass or Teflon, on fiberglass, or on woven mesh. In these systems, however, the convenience of catalyst immobilization is achieved at the expense of increased average convective-diffusion distance from fluid to catalyst surface.
Some research has focused on the use of inorganic ceramic membranes as a means by which to fix the photocatalyst. By immobilizing the photocatalyst in a membrane, molecules to be treated are brought into contact with the photocatalytic surface as the liquid passes through the membrane, thus allowing for continuous processing and decreased diffusion distances. U.S. Pat. No. 5,035,784, issued Jul. 10, 1991, describes the degradation of organic chemicals with a titanium ceramic membrane. However, this patent discloses a "membrane" comprised of a porous film coated on a non-porous glass substrate, such as beads or slides. Because the glass substrate is not porous, a constant concentration gradient is required to drive the contaminant reactants into the pores of the film. As the concentration of contaminant reactants decreases exponentially with time, the decomposition rate decreases. As a result, this system cannot completely eliminate the presence of contaminant reactants, and thus the effective efficiency of the system decreases over the duration of the reaction.
Surface illumination is another important aspect of photocatalysis. In U.S. Pat. No. 5,035,784 and in other work addressing the use of photocatalysis for decomposition of contaminants, large ultraviolet lamps are used to illuminate the photocatalyst, whether fixed on a support or as a slurry. Such lamps are limited in that they cannot effectively illuminate the membrane, particularly if the membrane is small (i.e. a few millimeters) and if in situ remediation is desired. The use of a large lamp in such a system also has disadvantages in that the system is not easily scaled up for in situ use.
In the above-referenced patent, a light delivery system comprising an optical fiber coated with a photocatalyst is also described. However, the use of a coated optical fiber has disadvantages in that the photocatalyst is coated on a non-porous support, and the contaminants cannot flow through such a support, thereby decreasing the efficiency of the process. Moreover, in this system, the photocatalytic coating is illuminated from the back, and thus, direct illumination of the surface of the photocatalyst is not possible. Such a configuration decreases the effectiveness of the process because fewer chemically active sites can be formed.
Accordingly, a need in the art exists for a photocatalytic reactor having at least one permeable photocatalytic membrane which permits either a gas or liquid fluid medium containing selected reactants to flow through the membrane so that the chemically active sites on the surface of the permeable photocatalytic membrane will have constant contact with the reactants and a light transmission system which directly illuminates the membrane, allows for in situ use of the photocatalytic reactor, and allows the reactor to be easily scaled to a particular application.